Generally, an image sensor refers to a semiconductor device which converts an optical image into an electric signal. Of such image sensors, a Charge Coupled Device (CCD) is a device in which respective Metal Oxide Semiconductor (MOS) capacitors are very close to each other and charge carriers are stored in and carried by the capacitors, and a Complementary metal-oxide semiconductor Imaging Sensor (CIS) refers to a device which employs a switching method. In the switching method, the number of MOS transistors are equal to the number of pixels using CMOS technology that employs a control circuit and a signal processing circuit as peripheral circuits, and the outputs are sequentially detected using the transistors. Since the CIS has a great advantage in that power consumption is small, the CIS is very useful for personal, portable systems such as mobile phones.
Meanwhile, one of the significant criteria for evaluating the quality of an image device is the dynamic range. The dynamic range generally represents a maximum range in which an input signal can be processed without distortion. For the image sensor, as the dynamic range thereof becomes wider, desired images can be acquired regardless of the amount of brightness variation.
FIG. 1 is a diagram showing a schematic circuit diagram of a conventional CMOS image sensor, which is compared with the present invention.
Referring to FIG. 1, a photodiode 101 functions to convert light energy into electric energy depending on light intensity. Transistors 103, 105, 107 and 109 transfer the electric image signal converted by the photodiode 101 to a comparator 111. A ramp signal 113 is a reference signal used to convert the electric signal generated by the photodiode 101 into a digital signal. The comparator 111 compares the ramp signal with the image signal generated by the photodiode 101 and digitizes the image signal. The digitized image signal is stored in memory 115.
However, the conventional image sensor has a narrow dynamic range, so that, when representing a very dark portion and a very bright portion in one image, as shown in FIGS. 14 and 15, the conventional image sensor has a disadvantage in that it cannot represent the dark portion or the bright portion, depending on the exposure time.